What does a long blast indicate while boating?

A long blast while boating serves as a warning signal to other vessels about the boat's movements, particularly when operating in narrow channels, fog, or areas where visibility is poor. The correct answer, indicating "Leaving dock," reflects a common maritime convention. When a vessel is leaving the dock or preparing to depart, a long blast warns nearby boats and individuals of its intentions, allowing them to take necessary precautions or keep clear.

The other options represent different signaling contexts. For instance, "going ahead" and "alter course to starboard" are typically indicated by different sound signals—short blasts, specifically. A distress signal involves a specific series of signals, often distinct, to communicate emergencies. Therefore, understanding the purpose of these sound signals is essential for safe navigation and communication among vessels on the water.